This invention relates to the reception of amplitude modulated radio signals, and more particularly to methods for reducing received noise in these signals.
The most general form of an broadcast AM signal is:
x(t)=[1+mi(t)]xc2x7cos xcfx89ct+mq(t)xc2x7sin xcfx89ct
where x(t) is the broadcast amplitude modulated RF signal, xcfx89c is the carrier frequency, mi(t) is the in-phase audio signal, and mq(t) is the quadrature audio signal, and where mi(t) and mq(t) are typically constrained:
xe2x88x921.0xe2x89xa6mi(t)xe2x89xa61.25 and |mq(t)|xe2x89xa61.0
For a conventional monophonic broadcast, mi(t) is the audio voltage of the program being broadcast, and mq(t) is nominally zero. At the receiver, an envelope detector may be used to recover an approximation to the mi(t) signal, called the E signal, or a synchronous detector may be used to recover mi(t) exactly, which is called the in-phase signal I. The various stereophonic AM broadcast systems use mi(t) and mq(t) differently, but typically, the left and right channel audio signals are summed and processed to create mi(t), and the left and right signals are subtracted and processed to create mq(t). In the receiver, synchronous detectors recover mi(t) and mq(t), to form the in-phase signal I and the quadrature phase signal Q, which are ultimately processed to yield the left and right channel audio signals.
An example of a stereo AM broadcast system is the CQUAM system (U.S. Pat. Nos. 4,218,586, 4,371,747). The sum and difference audio signals are predistorted at the transmitter to create mi(t) and mq(t). This distortion exactly corresponds to the distortion introduced when an envelope detector is used to demodulate the CQUAM broadcast signal. Thus, the typical mono receiver, which used an envelope detector, receives a low distortion mono signal. A CQUAM stereo receiver recovers the E, I, and Q signals, and uses the E and I signals to remove the distortion on the Q signal. The E and processed Q signals are summed and subtracted to create undistorted left and right channel audio signals.
There are many forms of noise, both natural and man-made, that may disrupt an AM broadcast. Natural noise, from cosmic and atmospheric sources can add broadband noise to the received signal. Noise from lightning discharges may add impulse like noise. Man-made noise may be broadband, or may have strong spectral components, depending upon its source.
One common source of man-made noise is synchronized to the AC power line frequency (60 Hz in North America). A typical example is a power transmission line having a faulty insulator which allows an electrical discharge once in each half cycle of the AC power waveform. The actual discharge is random in character, resulting in RF energy spread over a wide spectrum, even though the timing of the discharge is repetitive and stable from period to period.
When this sort of noise is added to an AM RF signal and demodulated, the resulting demodulated audio has a series of spike-like waveforms due to the noise. Whereas the timing of these spikes is typically very uniform and is synchronized with the AC power waveform, the amplitude and wave shape of the spikes vary with each discharge. Some spikes might be positive, and some might be negative. Some spikes may not be observable because their amplitude is so low compared to the broadcast program material. This variability is due to the random nature of the generated noise, and the random phase variations between each noise impulse and the carrier.
The result of this train of noise impulses is that a wide band noise signal is added to the demodulated audio signals. A useful model of this process is a white noise source that is gated on briefly at a rate synchronous with the AC power waveform and added to the demodulated audio. On an AM broadcast, this sort of RF noise results in an objectionable buzz-like noise in the demodulated audio. This type of noise is common, and in automobile reception may be especially obnoxious. The AC power lines parallel to the road act as a transmitting antenna, re-transmitting the noise from a single point noise source over an extensive length of road. Thus, what might have been a brief interruption to AM listening becomes an extended duration during which time reception is impaired or interrupted.
Typically, noise blankers have two components: one, which detects the presence of noise, and a second, which minimizes the audibility of the noise.
Noise blankers typically operate on the principle that normal modulation may not exceed +125% or xe2x88x92100%, whereas noise may. So, one commonly used approach senses the audio signal level at the output of the AM detector. When the magnitude of the output audio exceeds a level corresponding to modulation greater by some amount than 100%, the output level is clamped, or set to zero.
This approach may cause the duration of the noise impulse, and thus the duration for which the detector output is clamped, to be longer than necessary. This is because the finite bandwidth of the tuned stages in the receiver (typically dominated by the IF stages) causes the original noise pulse, which may have been quite brief, to be stretched in time by the low-pass action of the IF stages. Thus, the effect of even a brief noise impulse is comparable to the length of the impulse response of the low-pass filter equivalent of the IF pass band.
One approach that minimizes the effects of the pulse lengthening has been recently commercialized by Sprague Electric Company, in the form of a noise blanker IC (ULN3845A). This approach detects noise at the output of the RF amplifier stage, which is generally wideband. Because of the wideband nature of the signal at this point, the noise impulse has not experienced much lengthening. Noise impulses above some threshold level are detected, and the output of the RF amplifier is set to zero for the duration of the impulse, effectively blanking it. Since this interruption to the RF level may cause a disruption in the audio at the detector output, the detector output is sampled and held for the duration of the noise impulse. To compensate for the time delay and pulse stretching in the IF stages, the sample and hold control voltage is also delayed and stretched.
The invention has the advantage of providing a noise blanking system that operates on noise pulses substantially lower in level than xc2x1100% modulation. In addition, the invention provides a blanking action that is quite inaudible, doing little harm to the audio waveform. The system of the invention is effective on noise that is synchronized to the AC power line, and on other types of noise as well. Furthermore, when a receiver according to the invention is tuned to a weak station where a stronger station is closely located in frequency, it can still blank out noise pulses in the weak signal that are lower in amplitude than the RF stage""s 100% modulation level, which may be governed by the stronger signal. A combination synchronous-asynchronous noise reduction system can reduce noise even further than the synchronous system alone. The system of the invention may be advantageously combined with xe2x80x9cmonkey chatterxe2x80x9d reduction circuitry.